how/network

Routing and TTL

How does a packet reach Google from your laptop in 10-15 hops? Each router knows only "where to pass it next", and that is enough.

When you ping 8.8.8.8, the packet passes through 5-15 routers before it reaches Google. Odd: the internet is huge, so how do routers manage to "know the way"?

The simple answer: nobody knows "the whole map of the internet". Each router knows only its own little piece: "network X is on my interface Y, and everything else, hand it to that neighbor over there". This hop-by-hop logic is what makes the internet scale.

Press ▶ to watch one packet pass through 3 routers, how each of them makes a decision, and why TTL must be decremented.

step 1/5·00 · client prepares a packet for a foreign network

§ steps

A client with IP 10.0.0.5 wants to send a packet to 203.0.113.45. The stack looks in the routing-table:
- 10.0.0.0/24 dev eth0, our network
- default via 10.0.0.1, everything else goes through gateway R1
The address 203.0.113.45 does not fall under our /24, so we use the default-gateway. The packet leaves for R1 with a starting TTL=64 (the Linux default).

recap

What to remember:

Routing works hop-by-hop: no router knows "the whole way to the end", only the next step. Each one makes its own choice independently
TTL (Time To Live) protects against loops: each router decrements it by 1, and at TTL=0 the packet is dropped and an ICMP time-exceeded flies back to the sender. traceroute is built on this
Longest prefix match: if several routes in the table match the destination, the "longest" (most specific) one wins. /24 beats /16, /16 beats /0
Connected routes, networks physically attached to our interfaces, land in the routing-table automatically. You do not need to add them by hand
In Linux the table is ip route. You can play with routes without a reboot: add one with ip route add, remove one with ip route del

If you want to get your hands on it, there is a lab where a topology of 3 nodes is built and the packet travels through them with tcpdump observation.

§ dig into the knowledge base

§ try it hands-on

intermediate-03-three-node-routing- Three-node routing, build the topology by hand

§ steps

A client with IP 10.0.0.5 wants to send a packet to 203.0.113.45. The stack looks in the routing-table:

10.0.0.0/24 dev eth0, our network
default via 10.0.0.1, everything else goes through gateway R1

The address 203.0.113.45 does not fall under our /24, so we use the default-gateway. The packet leaves for R1 with a starting TTL=64 (the Linux default).